In integrated circuit technology, it is known to form devices in a semiconductor substrate or epitaxial layer because those formed on polysilicon films have poor characteristics. When a PN junction is formed directly in the substrate material it shares one terminal with other devices (generally referred to as ground) and cannot have both anode and cathode connected to arbitrary voltages. It is also known to form an "isolation region" around an integrated circuit device, in order to isolate the device electrically from other devices in the same substrate. The isolation region consists essentially of a "tub" shaped region of opposite conductivity to the substrate. Such a structure is shown in U.S. Pat. No. 5,156,989 to Williams et al. (see FIG. 10). When a diode is enclosed within an isolation region, depending on the voltage applied to the anode and cathode, a junction between the isolation region and the substrate may become forward-biased. This may cause minority carriers to be injected into the substrate, resulting in latch-up, excessive heating and other undesirable effects. It may also have parasitic bipolar transistors with high current gain, leading to large substrate current. In this sense, prior art diodes formed in a substrate cannot freely float.